1. Field of the Invention
The present invention relates to the surgical treatment of intervertebral discs in the lumbar, cervical, or thoracic spine that have suffered from tears in the anulus fibrosus, herniation of the nucleus pulposus and/or significant disc height loss.
2. Description of the Related Art
The disc performs the important role of absorbing mechanical loads while allowing for constrained flexibility of the spine. The disc is composed of a soft, central nucleus pulposus (NP) surrounded by a tough, woven anulus fibrosus (AF). Herniation is a result of a weakening in the AF. Symptomatic herniations occur when weakness in the AF allows the NP to bulge or leak posteriorly toward the spinal cord and major nerve roots. The most common resulting symptoms are pain radiating along a compressed nerve and low back pain, both of which can be crippling for the patient. The significance of this problem is increased by the low average age of diagnosis, with over 80% of patients in the U.S. being under 59.
Since its original description by Mixter & Barr in 1934, discectomy has been the most common surgical procedure for treating intervertebral disc herniation. This procedure involves removal of disc materials impinging on the nerve roots or spinal cord external to the disc, generally posteriorly. Depending on the surgeon's preference, varying amounts of NP are then removed from within the disc space either through the herniation site or through an incision in the AF. This removal of extra NP is commonly done to minimize the risk of recurrent herniation.
Nevertheless, the most significant drawbacks of discectomy are recurrence of herniation, recurrence of radicular symptoms, and increasing low back pain. Re-herniation can occur in up to 21% of cases. The site for re-herniation is most commonly the same level and side as the previous herniation and can occur through the same weakened site in the AF. Persistence or recurrence of radicular symptoms happens in many patients and when not related to re-herniation, tends to be linked to stenosis of the neural foramina caused by a loss in height of the operated disc. Debilitating low back pain occurs in roughly 14% of patients. All of these failings are most directly related to the loss of NP material and AF competence that results from herniation and surgery.
Loss of NP material deflates the disc, causing a decrease in disc height. Significant decreases in disc height have been noted in up to 98% of operated patients. Loss of disc height increases loading on the facet joints. This can result in deterioration of facet cartilage and ultimately osteoarthritis and pain in this joint. As the joint space decreases the neural foramina formed by the inferior and superior vertebral pedicles also close down. This leads to foraminal stenosis, pinching of the traversing nerve root, and recurring radicular pain. Loss of NP also increases loading on the remaining AF, a partially innervated structure that can produce pain. Finally, loss of NP results in greater bulging of the AF under load. This can result in renewed impingement by the AF on nerve structures posterior to the disc.
Persisting tears in the AF that result either from herniation or surgical incision also contribute to poor results from discectomy. The AF has limited healing capacity with the greatest healing occurring in its outer borders. Healing takes the form of a thin fibrous film that does not approach the strength of the uninjured disc. Surgical incision in the AF has been shown to produce immediate and long lasting decreases in stiffness of the AF particularly against torsional loads. This may over-stress the facets and contribute to their deterioration. Further, in as many as 30% of cases, the AF never closes. In these cases, not only is re-herniation a risk but also leakage of fluids or solids from within the NP into the epidural space can occur. This has been shown to cause localized pain, irritation of spinal nerve roots, decreases in nerve conduction velocity, and may contribute to the formation of post-surgical scar tissue in the epidural space.
Other orthopedic procedures involving removal of soft tissue from a joint to relieve pain have resulted in significant, long lasting consequences. Removal of all or part of the menisci of the knee is one example. Partial and total meniscectomy leads to increased osteoarthritic degeneration in the knee and the need for further surgery in many patients. A major effort among surgeons to repair rather than resect torn menisci has resulted in more durable results and lessened joint deterioration.
Systems and methods for repairing tears in soft tissues are known in the art. One such system relates to the repair of the menisci of the knee and is limited to a barbed tissue anchor, an attached length of suture, and a suture-retaining member, which can be affixed to the suture and used to draw the sides of a tear into apposition. The drawback of this method is that it is limited to the repair of a tear in soft tissue. In the intervertebral disc, closure of a tear in the AF does not necessarily prevent further bulging of that disc segment toward the posterior neural elements. Further, there is often no apparent tear in the AF when herniation occurs. Herniation can be a result of a general weakening in the structure of the AF (soft disc) that allows it to bulge posteriorly without a rupture. When tears do occur, they are often radial.
Another device known in the art is intended for repair of a tear in a previously contiguous soft tissue. Dart anchors are placed across the tear in a direction generally perpendicular to the plane of the tear. Sutures leading from each of at least two anchors are then tied together such that the opposing sides of the tear are brought together. However, all of the limitations pertaining to repair of intervertebral discs, as described above, pertain to this device.
Also known in the art is an apparatus and method of using tension to induce growth of soft tissue. The known embodiments and methods are limited in their application to hernias of the intervertebral disc in that they require a spring to apply tension. Aside from the difficulty of placing a spring within the limited space of the intervertebral disc, a spring will induce a continuous displacement of the attached tissues that could be deleterious to the structure and function of the disc. A spring may further allow a posterior bulge in the disc to progress should forces within the disc exceed the tension force applied by the spring. Further, the known apparatus is designed to be removed once the desired tissue growth has been achieved. This has the drawback of requiring a second procedure.
There are numerous ways of augmenting the intervertebral disc disclosed in the art. In reviewing the art, two general approaches are apparent—implants that are fixed to surrounding tissues and those that are not fixed, relying instead on the AF to keep them in place.
The first type of augmenting of the intervertebral disc includes generally replacing the entire disc. This augmentation is limited in many ways. First, by replacing the entire disc, they generally must endure all of the loads that are transferred through that disc space. Many degenerated discs are subject to pathologic loads that exceed those in normal discs. Hence, the designs must be extremely robust and yet flexible. None of these augmentation devices has yet been able to achieve both qualities. Further, devices that replace the entire disc must be implanted using relatively invasive procedures, normally from an anterior approach. They may also require the removal of considerable amounts of healthy disc material including the anterior AF. Further, the disclosed devices must account for the contour of the neighboring vertebral bodies to which they are attached. Because each patient and each vertebra is different, these types of implants must be available in many shapes and sizes.
The second type of augmentation involves an implant that is not directly fixed to surrounding tissues. These augmentation devices rely on an AF that is generally intact to hold them in place. The known implants are generally inserted through a hole in the AF and either expand, are inflated, or deploy expanding elements so as to be larger than the hole through which they are inserted. The limitation of these concepts is that the AF is often not intact in cases requiring augmentation of the disc. There are either rents in the AF or structural weaknesses that allow herniation or migration of the disclosed implants. In the case of a disc herniation, there are definite weaknesses in the AF that allowed the herniation to occur. Augmenting the NP with any of the known augmentation devices without supporting the AF or implant risks re-herniation of the augmenting materials. Further, those devices with deployable elements risk injuring the vertebral endplates or the AF. This may help to retain the implant in place, but again herniations do not require a rent in the AF. Structural weakness in or delamination of the multiple layers of the AF can allow these implants to bulge toward the posterior neural elements. Additionally, as the disc continues to degenerate, rents in the posterior anulus may occur in regions other than the original operated site. A further limitation of these concepts is that they require the removal of much or all of the NP to allow insertion of the implant. This requires time and skill to achieve and permanently alters the physiology of the disc.
Implanting prostheses in specific locations within the intervertebral disc is also a challenging task. The interior of the disc is not visible to the surgeon during standard posterior spinal procedures. Very little of the exterior of the disc can be seen through the small window created by the surgeon in the posterior elements of the vertebrae to gain access to the disc. The surgeon further tries to minimize the size of any anulus fenestration into the disc in order to reduce the risk of postoperative herniation and/or further destabilization of the operated level. Surgeons generally open only one side of the posterior anulus in order to avoid scarring on both sides of the epidural space.
The rigorous requirements presented by these limitations on access to and visualization of the disc are not well compensated for by any of the intradiscal prosthesis implantation systems currently available.
The known art relating to the closure of body defects such as hernias through the abdominal wall involve devices such as planer patches applied to the interior of the abdominal wall or plugs that are placed directly into the defect. The known planar patches are limited in their application in the intervertebral disc by the disc's geometry. The interior aspect of the AF is curved in multiple planes, making a flat patch incongruous to the surface against which it must seal. Finally, the prior art discloses patches that are placed into a cavity that is either distended by gas or supported such that the interior wall of the defect is held away from internal organs. In the disc, it is difficult to create such a cavity between the inner wall of the anulus and the NP without removing nucleus material. Such removal may be detrimental to the clinical outcome of disc repair.
One hernia repair device known in the art is an exemplary plug. This plug may be adequate for treating inguinal hernias, due to the low pressure difference across such a defect. However, placing a plug into the AF that must resist much higher pressures may result in expulsion of the plug or dissection of the inner layers of the anulus by the NP. Either complication would lead to extraordinary pain or loss of function for the patient. Further, a hernia in the intervertebral disc is likely to spread as the AF progressively weakens. In such an instance, the plug may be expelled into the epidural space.
Another hernia repair device involves a curved prosthetic mesh for use in inguinal hernias. The device includes a sheet of material that has a convex side and a concave side and further embodiments with both spherical and conical sections. This device may be well suited for inguinal hernias, but the shape and stiffness of the disclosed embodiments are less than optimal for application in hernias of the intervertebral disc. Hernias tend to be broader (around the circumference of the disc) than they are high (the distance between the opposing vertebrae), a shape that does not lend itself to closure by such conical or spherical patches.
Another device involves an inflatable, barbed balloon patch used for closing inguinal hernias. This balloon is left inflated within the defect. A disadvantage of this device is that the balloon must remain inflated for the remainder of the patient's life to insure closure of the defect. Implanted, inflated devices rarely endure long periods without leaks, particularly when subjected to high loads. This is true of penile prostheses, breast implants, and artificial sphincters.
Another known method of closing inguinal hernias involves applying both heat and pressure to a planar patch and the abdominal wall surrounding the hernia. This method has the drawback of relying entirely on the integrity of the wall surrounding the defect to hold the patch in place. The anulus is often weak in areas around a defect and may not serve as a suitable anchoring site. Further, the planar nature of the patch has all of the weaknesses discussed above.
Various devices and techniques have further been disclosed for sealing vascular puncture sites. The most relevant is a hemostatic puncture-sealing device that generally consists of an anchor, a filament and a sealing plug. The anchor is advanced into a vessel through a defect and deployed such that it resists passage back through the defect. A filament leading from the anchor and through the defect can be used to secure the anchor or aid in advancing a plug that is brought against the exterior of the defect. Such a filament, if it were to extend to the exterior of the disc, could lead to irritation of nerve roots and the formation of scar tissue in the epidural space. This is also true of any plug material that may be left either within the defect or extending to the exterior of the disc. Additionally, such devices and methods embodied for use in the vascular system require a space relatively empty of solids for the deployment of the interior anchor. This works well on the interior of a vessel, however, in the presence of the more substantial NP, the disclosed internal anchors are unlikely to orient across the defect as disclosed in their inventions.
As described above, various anulus and nuclear augmentation devices have been disclosed in the art. The prior art devices, however, suffer from multiple limitations that hinder their ability to work in concert to restore the natural biomechanics of the disc. The majority of nuclear augmentation prostheses or materials function like the nucleus and transfer most of the axial load from the endplates to the anulus. Accordingly, such augmentation materials conform to the anulus under loading to allow for load transmission from the endplates. In this type of intervention, however, the cause of the diminished nucleus pulposus content remains untreated. A disc environment with a degenerated anulus, or one having focal or diffuse lesions, is incapable of maintaining pressure to support load transmission from either the native nucleus or a prosthetic augmentation and will inevitably fail. In these cases, such augmentation prostheses can bulge through defects, extrude from the disc, or apply pathologically high load to damaged regions of the anulus.